Dumb question about SmartCPM — why does plain CPM still exist?

[Danny12345]

Sorry if this is a dumb question, but it's about SmartCPM.

On PropellerAds I read that SmartCPM is a bidding model that saves you money.
But here's what I can't wrap my head around: if I set both plain CPM and SmartCPM to $0.5,
then SmartCPM should always end up cheaper
— or at worst, cost exactly the same as plain CPM. It can never cost more.

So wouldn't everyone just pick SmartCPM every single time?
If that's true, why does PropellerAds even keep a plain CPM bidding option?
Why not just offer SmartCPM only?

Thanks in advance — genuinely curious what I'm missing here.